1. Field of the Invention
The present invention relates generally to elastomeric bearings, and more particularly to tether bearings for risers and tendons for offshore platforms.
2. Background Art
For offshore drilling operations, it is conventional to moor a floating marine platform by tendons depending from the platform to anchors on the seabed. In such a tendon leg platform (TLP), the tendons hold the platform at a level below its normal buoyancy level in order to reduce vertical buoyant movement of the platform. Therefore, the tendons are put under tensile stress by the buoyancy of the platform. Elastomeric bearings are often used for connecting the tendons to the floating platform in order to reduce tension variation caused by the vertical buoyant movement of the platform and to accommodate some pivoting and axial rotation of the tendons with respect to the floating platform.
A final tension adjustment of the tendons of a TLP is usually made after deballasting of the TLP hull. As a result of deballasting, the gross tendon load is applied via vessel buoyancy. A final adjustment to equalize the load in all tendons is made by mechanical means incorporated into a top connector of each tendon. The final adjustment is accomplished by ballasting the TLP to remove load from the tendons, adjusting the lengths of the tendons, and then deballasting again.
In order to precisely measure the load in all of the tendons, load cells can be interposed between the top connector of each tendon and the TLP. Transducers such as strain gages measure the force applied to the load cells. However, the use of such load cells create structural problems since their inclusion in the load path requires massive support rings to prevent distortions. These support structures can also create stress risers in the conduit string.
Tensioned drilling risers, production risers, and export risers may also be suspended from floating platforms. For such a riser it is desirable to measure the tension applied to the riser during an initial adjustment and to monitor the tension during use in order to ensure that the tension remains within desired design limits.